BUG: fix romb in scipy.integrate.quadrature

Routine romb for Romberg integration using 2**k +1 samples of a function
was only using 2**(k-1) +1 samples due to wrong ranges.
Renumbered the values R to the common textbook numbering scheme starting
with 0s. Note that romb with 2**k +1 should give the same values as
romberg with divmax=k. Or with 3 values romb is the same as simps.
DOC: use MathJax in html docs
MAINT: docs: correct location of the html files

`make html-scipyorg` places files into doc/build/html-scipyorg
not doc/build/html
ENH: fitpack2: add _from_tck classmethod to BivariateSpline

similar to the _from_tck to UnivariateSpline
ENH: interpolate: add the feature of constant extrapolation to 1d spline.
TST: interpolate: tweak the extrap modes tests
MAINT: Drive-by PEP8/pyflakes fixes.
DOC: surround methods in class docstrings with backticks.

Fixes #3915.
Changed doc for romb's dx parameter to int.

See `cumtrapz`, `simps`, etc.

Closes #3721
Added a warning about leastsq using with NaNs.

Added a warning that `func`'s retuned array_like must not contain NaNs or
computation might fail. Closes #3491
changed int to float in docstring
TST: integrate: add test for romb accuracy improvement
Merge pull request #3731 from 'pr-3731'

BUG: interpolate: fix wrong romb sample usage

Routine romb for Romberg integration using 2**k +1 samples of a function
was only using 2**(k-1) +1 samples due to wrong ranges.  Renumbered the
values R to the common textbook numbering scheme starting with 0s.
Merge pull request #3927 from venthur/fix-3721

DOC: integrate: Changed doc for romb's dx parameter to float
Merge pull request #3929 from venthur/fix-3491

DOC: optimize: Added a warning about leastsq using with NaNs
DOC: remove `import scipy as sp` abbreviation here and there
Merge pull request #3933 from ev-br/scipy_as_sp

DOC: remove `import scipy as sp` abbreviation here and there
Merge pull request #3926 from jvkersch/doc-stats-3915

DOC: Surround stats class methods in docstrings with backticks.
TST: fitpack: remove the docstring from a test function
Merge pull request #3917 from ev-br/pr-3697-1

splines, constant extrapolation